Displacement control hydrostatic propulsion system for multirotor  vertical take off and landing aircraft

ABSTRACT

A hydraulic propulsion system is disclosed. The system includes one or more input interfaces configured to receive mechanical power from a power source, four or more variable displacement pumps coupled to the one or more input interfaces adaptable to generate a controlled variable quantity of fluid to be pumped out of each of the variable displacement pumps in response to a control input from a corresponding control interface, and four or more positive displacement motors each fluidly coupled to a corresponding variable displacement pump and configured to receive the pumped fluid, wherein each motor is configured to be mechanically coupled to one or more aerodynamic rotors of a multi-rotor vertical take-off and landing aircraft to control thrust and attitude.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is related to and claims the prioritybenefit of U.S. Provisional Patent Application Ser. No. 62/571,183 filedOct. 11, 2017, U.S. Provisional Patent Application Ser. No. 62/571,192filed Oct. 11, 2017, and a counterpart international application to befiled the same day as the present disclosure having the title AviationHydraulic Propulsion System Utilizing Secondary Controlled Drives, thecontents of each of which are hereby incorporated by reference in theirentirety into the present disclosure.

TECHNICAL FIELD

The present disclosure relates to a hydraulic propulsion system forrotary-wing aircrafts.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Multi-rotor vertical take-off and landing (VTOL) aircrafts are becomingcommonplace. These VTOL aircrafts have to be equipped with a minimum offour independently controlled rotors to control motion of the aircraftincluding pitch, roll, and yaw as well as attitude and translationalmotion without inclusion of any other motion controlling device. Twoimportant limitations associated with these aircrafts include weight andability to independently control each rotor. Various propulsion systemsare used, such as electrical, mechanical, and electromechanical.However, each suffer from excessive weight and/or lack of responsivenesslimiting their utility. In particular, in order to dynamically controleach rotor independently so that a desired attitude can be achieved forthe aircraft, a number of complicated devices are typically used whichare both heavy and require constant maintenance.

Therefore, there is an unmet need for a novel approach for propulsion ofVTOL aircrafts.

SUMMARY

A hydraulic propulsion system is disclosed. The system includes one ormore input interfaces configured to receive mechanical power from apower source, four or more variable displacement pumps coupled to theone or more input interfaces adaptable to generate a controlled variablequantity of fluid to be pumped out of each of the variable displacementpumps in response to a control input from a corresponding controlinterface, and four or more positive displacement motors each fluidlycoupled to a corresponding variable displacement pump and configured toreceive the pumped fluid. Each motor is configured to be mechanicallycoupled to one or more aerodynamic rotors of a multi-rotor verticaltake-off and landing aircraft to control thrust and attitude.

According to one embodiment of the system, the power source is one ormore internal combustion engines.

According to one embodiment of the system, the power source is one ormore electric motors.

According to one embodiment of the system, the power source is one ormore turbine engines.

According to one embodiment of the system, the positive displacementmotors are fixed displacement motors.

According to one embodiment of the system, the positive displacementmotors are variable displacement motors adapted to further change therotor speed for the corresponding pump fluid flow and wherein the motordisplacement is controlled by a motor displacement control device whichis one of an electro-hydraulic displacement control device, mechanicaldisplacement control device, electro-mechanical displacement controldevice, and a combination thereof.

According to one embodiment of the system, the quantity of fluid to bepumped out of each of the variable displacement pumps is controlled by apump displacement control device which is one of an electro-hydraulicdisplacement control device, mechanical displacement control device,electro-mechanical displacement control device, and a combinationthereof.

According to one embodiment of the system, the control input is providedfrom one of a flight control computer, a pilot, and a combinationthereof.

According to one embodiment of the system, the system further includes aflight control computer coupled to each of the variable displacementpumps and configured to control the quantity of fluid to be pumped fromeach.

According to one embodiment of the system, the system is furtherconfigured to receive a signal corresponding to the speed of each motorand to provide the speed information as speed feedback signals to theflight control computer.

According to one embodiment of the system, the system further includes aclosed-loop control arrangement using the speed feedback signals.

According to one embodiment of the system, the system is furtherconfigured to receive a signal corresponding to the displacement of eachpositive displacement motors and to provide the displacement informationas displacement feedback signals to the flight control computer.

According to one embodiment of the system, the system further includes aclosed-loop control arrangement using the displacement feedback signals.

According to one embodiment of the system, the flight control computerfurther configured to receive signals corresponding to one or more ofposition, attitude, and motion of the aircraft and control fluid flowfrom the variable displacement pumps accordingly to achieve a desiredposition, attitude and motion of the aircraft.

According to one embodiment of the system, the system further includes acharge pump adapted to provide power for the pump displacement controldevice.

According to one embodiment of the system, the system further includes afluid cooling device adapted to cool fluid used therein.

According to one embodiment of the system, the variable displacementpumps are coupled to each other in series manner.

According to one embodiment of the system, the variable displacementpumps are coupled in pairs in a series manner, and each pair is coupledto at least one other pair in a parallel manner.

According to one embodiment of the system, each pair is coupled to adedicated power source.

According to one embodiment of the system, each pair is coupled to theone or more input interfaces.

According to one embodiment of the system, the one or more inputinterfaces is a gearbox.

According to one embodiment of the system, each pair is coupled to adedicated input interface which is coupled to a dedicated power source.

According to one embodiment of the system, the fluid is a compressiblefluid.

According to one embodiment of the system, the compressible fluid isair.

According to one embodiment of the system, the fluid is anincompressible fluid.

According to one embodiment of the system, the incompressible fluid isone of hydraulic oil, water, fuel, antifreeze, and a combinationthereof.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic representations of directional movementsof an aircraft from a top view (FIG. 1A) and a side view (FIG. 1B).

FIG. 2A is a schematic of an embodiment of a hydraulic propulsion systemaccording to the present disclosure.

FIG. 2B is a schematic of another embodiment of a hydraulic propulsionsystem according to the present disclosure.

FIG. 2C is a schematic of yet another embodiment of a hydraulicpropulsion system according to the present disclosure.

FIG. 2D is a schematic of yet another embodiment of a hydraulicpropulsion system according to the present disclosure.

FIGS. 3A and 3B are schematics of a top view (FIG. 3A) and a side view(FIG. 3B) of an exemplary embodiment of the hydraulic propulsion systemof any one of FIG. 2A, 2B, 2C, or 2D.

FIGS. 3C and 3D are top view schematics of alternative embodiments withsix rotors (FIG. 3C) and eight rotors (FIG. 3D).

FIGS. 3E and 3F are side view schematics of alternative embodiments withfour rotors, according to the present disclosure.

FIGS. 3G, 3H, and 3I are side view schematics of alternative embodimentsbased on one or more fixed displacement motors driving one or morerotors directly or via a gearbox.

FIG. 4A is a schematic of a propulsion control system according to thepresent disclosure which can be used in conjunction with one or more ofthe embodiments disclosed herein.

FIG. 4B is a schematic of another propulsion control system according tothe present disclosure which can be used in conjunction with one or moreof the embodiments disclosed herein.

FIG. 4C is a control block diagram according to the present disclosurewhich can be used in conjunction with one or more of the embodimentsdisclosed herein.

FIG. 5 is a schematic of another embodiment of a hydraulic propulsionsystem according to the present disclosure.

FIG. 6A is a schematic of another propulsion control system according tothe present disclosure which can be used in conjunction with one or moreof the embodiments disclosed herein.

FIG. 6B is a schematic of another propulsion control system according tothe present disclosure which can be used in conjunction with one or moreof the embodiments disclosed herein.

FIG. 6C is a control block diagram according to the present disclosurewhich can be used in conjunction with one or more of the embodimentsdisclosed herein.

FIG. 7 is a schematic of another embodiment of a hydraulic propulsionsystem according to the present disclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure, the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

The propulsion system according to the present disclosure is related toa family of multi-rotor aircrafts that include at least four rotorswhich are independently speed controlled. The propulsion systemaccording to the present disclosure is used to control the differentspeeds of a minimum of four rotors to achieve motion in pitch, roll, andyaw directions as shown in FIGS. 1A and 1B. The thrust generated fromthe aerodynamic rotors is also used to overcome gravity and drag and tocontrol the attitude of the aircraft. In FIGS. 1A and 1B, schematics ofa top view and side view, respectively, of the aircraft is shown. Yaw isdefined as the rotational movement about a z-axis that passes verticallythrough the center of the aircraft. Roll is defined as the rotationalmovement about an x-axis that passes horizontally through the center ofthe aircraft. Pitch is defined as the rotational movement about a y-axisthat passes horizontally through the center of the aircraft.

The propulsion system according to the present disclosure utilizes atleast four hydrostatic transmissions to distribute and transmitmechanical power from a power source (e.g., single or multiple internalcombustion engines, or turbine engines, or electric motors) to therotors (which can be single or multiple propellers, fans, orcompressors). Each hydrostatic transmission contains a variabledisplacement hydraulic pump, at least one fixed or variable displacementmotor, and the pipes and hoses that connect the pump and the motor. Therotor speed is controlled by the displacement of the pump. Thedisplacement of the pump is controlled by a control arrangementincluding electrical, mechanical, electromechanical, hydraulic,electrohydraulic, mechanical-hydraulic actuators, by human power throughappropriate linkages, or any combination thereof, further describedbelow.

This hydrostatic propulsion system of the present disclosure isconfigured to control the speed of each individual rotor with fasterresponse and lower weight in comparison with prior art propulsion systemcounterpart owing to the bandwidth of the displacement control and thecompactness of hydraulic units. As a result, a more stable flight andmore useful payload capability can be achieved. The reliability of theaircraft increases due to the highly reliable nature of hydraulicsystems. Furthermore, the aircraft power source (e.g., internalcombustion engine) can be arranged to run at relatively constant speed,which extends the lifetime of such power source. Furthermore, sincehydraulic components are made of metal, the propulsion system of thepresent disclosure can be made with less cost and is further readilyrecyclable.

The present disclosure is related to a counterpart application to befiled the same day as the present disclosure having the title AviationHydraulic Propulsion System Utilizing Secondary Controlled Drives. Thedifference between the present disclosure and this counterpartapplication lies in control strategies. In the present disclosure thecontrol scheme is based on primary or displacement control in which thehydraulic propulsion system controls the rotational speed of thehydraulic motor by changing the pump displacement. As such, the motorscan be fixed displacement or variable displacement. In case of usingvariable displacement motor, the motor displacement changes only toassist the pump to achieve improved overall performance. In thedisplacement control hydraulic propulsion system of the presentdisclosure, the bandwidth of the thrust is substantially determined bythe bandwidth of the pump. According to the present disclosure, in orderto control four propeller speeds independently, at least 4 pumps and 4motors are required for the displacement control hydraulic propulsionsystem. In contrast, the disclosure found in the counterpart applicationis based on a secondary control hydraulic propulsion system whichcontrols the output (i.e., speed of the propellers) by changing themotor displacement. As such, the pumps can be fixed displacement orvariable displacement. In case of using fixed displacement pump, thesystem pressure is adjusted by utilizing a valve network. In case ofusing variable displacement pump, the pump displacement changes toadjust the system pressure. In the secondary control hydraulicpropulsion system, the bandwidth of the thrust is substantiallydetermined by the bandwidth of the motor. In case of multiple motors,each motor speed can be controlled independently. Therefore, in order tocontrol, e.g., 4 propeller speeds independently, at least 1 pump and 4motors are required for the secondary control hydraulic propulsionsystem. Comparing to the counterpart application, the present disclosurebenefits from the control strategy simplicity and the lightweight owingto the fixed displacement motors.

Referring to FIG. 2A, a schematic of an embodiment of a propulsionsystem 1000A according to the present disclosure is shown. An engine 48is mechanically coupled and powers directly via a driveshaft 50 fourvariable displacement pumps 35, 36, 37, and 38 disposed in a seriesmanner such that rotational speed of each pump 35-38 (also, thedriveshaft 50) is substantially the same. IT should be appreciated thatwhile an engine (engine 48) is shown in FIG. 2A, other power sourcessuch as electric motors, can be substituted for the engine 48. Hydrauliclines 39, 40, 41, 42, 43, 44, 45, and 46 couple the variabledisplacement pumps 35, 36, 37, and 38 with fixed displacement motors 7,8, 9, and 10. Each motor 7, 8, 9, and 10 is in mechanical communicationand drives aerodynamic rotors 3, 4, 5, and 6 which convert powertransmitted by the driveshaft 50 into lift and thrust. Each variabledisplacement pump 35, 36, 37, and 38 can be a positive displacementmachine such as an axial, radial piston or vane-type machine, orcombinations thereof which generally comprise an array of displacementelements arranged radial or axial to the driveshaft 50. For example, inaxial piston machines, one or more piston-cylinder combinations arearranged axially in a cylinder block. An example is shown in USpublication 20120079936 for Ivantysynova et al., incorporated byreference herein in its entirety. For example, by linear movement of apiston within a cylinder, the piston which is coupled to a swash platecontrols the swash plate angle which controls the output flow of thevariable displacement pump which can in turn control the speed of thefixed displacement motor, to thereby control the speed of theaerodynamic rotor driven by the fixed displacement motor. By adjustingthe inclination of the swash plate to one extreme, e.g., a verticalposition, the displacement of the pump can be decreased to about zero.The angle of the swash plate can be controlled by electrical,mechanical, electromechanical, hydraulic, electrohydraulic,mechanical-hydraulic actuators, human power though appropriate linkages,or any combination thereof. By adjusting the swash plate inclinationindependently and separately, each of the variable displacement pumps35, 36, 37, and 38 can be configured to communicate different amounts offlow to a respective fixed displacement motor 7, 8, 9, and 10 causingeach of the coupled rotors 3, 4, 5, and 6 to rotate at a desired speed.By doing so, the desired directional pitch, roll, and yaw andtranslational motion can be achieved such that the aircraft can movefrom a Cartesian position X1Y1Z1 with a movement defined by a vector A1to a Cartesian position X2Y2Z2 with a movement defined by a vector A2.

Referring to FIG. 2B, a schematic of another embodiment of a propulsionsystem 1000B according to the present disclosure is provided. Theembodiment shown in FIG. 2B differs from the embodiment shown in FIG.2A, in that the engine 48 is coupled to a gearbox 47 via a driveshaft 49prior to being coupled to the driveshafts 50A and 50B. Additionally, thevariable displacement pumps 37 and 38 are provided in series and thecombination is provided in parallel to the series coupled variabledisplacement pumps 35 and 36 such that rotational speed of the driveshaft 50A is the same for the variable displacement pumps 37 and 38 andthe rotational speed of the drive shaft 50B is the same for the variabledisplacement pumps 35 and 36. Given the gear ratios of the gearbox 47,rotational speed of driveshafts 50A and 50B can be set to besubstantially the same or different. By adjusting the displacement ofeach pump 35, 36, 37, and 38 the propulsion system 1000B can beconfigured to communicate different amounts of flow to a respectivefixed displacement motor 7, 8, 9, and 10 causing each of the coupledrotors 3, 4, 5, and 6 to rotate at a desired/selective speed.

Referring to FIG. 2C, a schematic of yet another embodiment of apropulsion system 1000C according to the present disclosure is provided.The embodiment shown in FIG. 2C differs from the embodiment shown inFIG. 2A, in that two engines 48A and 48B are coupled to the driveshafts50A and 50B which are coupled to the variable displacement pumps 37 and38 which are provided in series and the combination is provided inparallel to the series coupled variable displacement pumps 35 and 36such that rotational speed of the drive shaft 50A is the same forvariable displacement pumps 37 and 38 and the rotational speed of thedrive shaft 50B is the same for the variable displacement pumps 35 and36. The rotational speed of driveshafts 50A and 50B can be set to besubstantially the same or different by operating the respective engines48A and 48B at substantially the same or different speeds. By adjustingthe displacement of each variable displacement pump 35, 36, 37, and 38the propulsion system 1000C can be configured to communicate differentamounts of flow to a respective fixed displacement motor 7, 8, 9, and 10causing the coupled rotor 3, 4, 5, and 6 to each rotate at adesired/selective speed. In FIG. 2C, each engine powers part of thepropulsion system, advantageously allowing placement of the engines 48Aand 48B and the variable displacement pumps 37, 38 and 35, 36 closer tothe fixed displacement motors 7, 8 and 9, 10, and the propeller 3, 4 and5, 6, respectively.

Referring to FIG. 2D, a schematic of yet another embodiment of apropulsion system 1000D according to the present disclosure is provided.The embodiment shown in FIG. 2D differs from the embodiment shown inFIG. 2B, in that two engines 48A and 48B are coupled to a gearbox 47 viadriveshafts 49A and 49B prior to being coupled to the driveshafts 50Aand 50B, respectively. These driveshafts are in turn coupled to thevariable displacement pumps 37 and 38 which are provided in series andthe combination is provided in parallel to the series coupled variabledisplacement pumps 35 and 36 such that rotational speed of the driveshaft 50A is the same for pumps 37 and 38 and the rotational speed ofthe drive shaft 50B is the same for pumps 35 and 36. The rotationalspeed of driveshafts 50A and 50B can be set to be substantially the sameor different by operating the respective engines 48A and 48B atsubstantially the same or different speeds or by choosing different gearratios in the gearbox 47. By adjusting the displacement of each variabledisplacement pump 35, 36, 37, and 38 the propulsion system 1000D can beconfigured to communicate different amounts of flow to the respectivefixed displacement motor 7, 8, 9, and 10 causing the coupled rotor 3, 4,5, and 6 to each rotate at a desired/selective speed. In FIG. 2D, allthe engines power the propulsion system together The advantage of thepropulsion system 1000D is the redundancy of multiple engines such thatwhen one engine 48A or 48B has experienced a complete or partial enginefailure, the propulsion system will work at s reduced power level.

Referring to FIGS. 3A and 3B, schematics of a top view and a side viewof an exemplary embodiment of the propulsion system 1000A, 1000B, 1000C,or 1000D according to the present disclosure are provided. Fourindividually controlled rotors 3, 4, 5, and 6 are driven by four fixdisplacement motors 7, 8, 9, and 10. The center compartment 2 containsthe common components that are shared by all the rotors, such as aflight control computer and a shared power source. The payload may alsobe located at the center of the aircraft. For manned aircraft, thecenter compartment 2 also includes the cockpit and the cabin (notshown). The aircraft is further defined by a frame 31.

The propulsion system according to the present disclosure can also havemore than four rotors. Schematics of exemplary embodiments with 6 and 8rotors are shown in FIGS. 3C and 3D, respectively. In addition to rotors3, 4, 5, and 6, in FIG. 3C rotors 11 and 12 are also provided. Rotors 11and 12 are coupled to motors 13 and 14, in a similar manner as discussedabove. In addition to rotors 3, 4, 5, 6, 11, and 12, in FIG. 3D rotors15 and 16 are also provided. Rotors 15 and 16 are coupled to motors 17and 18 in a similar manner as discussed above. Other implementationswith different number of rotors are also possible. For example, 5 or 7rotors may be implemented, or more than 8 rotors may be implemented in asimilar fashion.

Besides the arrangements shown in FIGS. 3C and 3D, other arrangementswith respect to the position of the rotors may also be possible.Referring to FIGS. 3E and 3F schematics of different rotor layouts areprovided as alternative layouts to those shown in other exemplaryembodiments of the propulsion system of the present disclosure. Inaddition to motor 8 being coupled to rotor 4, representing a first setof motor-rotor combination, in FIG. 3E, motor 24 is also provided and iscoupled to rotor 20, representing a second set of motor-rotorcombination. In addition, motor 9 being coupled to rotor 5, representinga third set of motor-rotor combination, in FIG. 3E, motor 25 is alsoprovided and is coupled to rotor 21, representing a fourth set ofmotor-rotor combination. Alternatively, in addition to motor 8 beingcoupled to rotor 4, representing a first set of motor-rotor combination,in FIG. 3F, motor 32 is also provided and is coupled to rotor 28,representing a second set of motor-rotor combination, such that the twosets are coupled to each other in a parallel manner. In addition, motor9 being coupled to rotor 5, representing a third set of motor-rotorcombination, in FIG. 3F, motor 33 is coupled to rotor 29, representing afourth set of motor-rotor combination, such that the third and fourthsets are coupled to each other in a parallel manner.

In another embodiment, according to the present disclosure, withreference to FIG. 3G, a schematic is provided of a single fixdisplacement motor 7 driving two or more rotors 58 and 59 via a gearbox54.

In yet another embodiment, according to the present disclosure, withreference to FIG. 3H, a schematic is provided of two or more fixdisplacement motors 7 and 66 coupled in series driving one rotor 3.

In still yet another embodiment, according to the present disclosure,with reference to FIG. 3I, a schematic is provided of one fixdisplacement motor 7 driving one rotor 3 via a gearbox 67.

With reference to FIG. 4A, a schematic of a propulsion system 100Aaccording to the present disclosure is provided that can be used inconjunction with one or more of the embodiments disclosed herein. Thesystem of FIG. 4A is a closed circuit system with an open-loop speedcontrol provision. A single or multiple variable displacement pumps 35are used to control flow to a single or multiple hydraulic fixeddisplacement motors 7 to control the speed of the aerodynamic rotor 3.As described above, the speed of the hydraulic motor 7 and theaerodynamic rotor 3 is controlled by the displacement of the pump 35which is driven by the engine 48 via a shaft 49 and a distributinggearbox 47. A displacement control device 70 is used to adjust the pumpdisplacement according to a command signal generated from the pilot or aflight control computer in an open-loop manner. According to oneembodiment of the present disclosure, the hydraulic drive of single ormultiple rotors contains an optional cooling device 72 for cooling thehydraulic fluid used therein. In FIG. 4A, a reservoir 71 is shown influid communication with the low-pressure side of the system. Thereservoir 71 can be a pressurized tank, bootstrap reservoir, or alow-pressure hydro-pneumatic accumulator.

With reference to FIG. 4B, a schematic of a propulsion system 100Baccording to the present disclosure is provided that can also be used inconjunction with one or more of the embodiments disclosed herein.Differences between FIGS. 4B and 4A include a charge pump 85 and valve87. The charge pump 85 can also be used to provide the power for thedisplacement control device 70 of the variable displacement pump 35.According to the embodiment shown in FIG. 4B, a charge pump 85 is usedwith each variable displacement pump 35; or in some cases, a charge pump85 is shared by a number of variable displacement pumps 35. Valve 87isolates the charge pump 85 from the hydraulic motor and preventsunwanted rotation of the rotor. In any of these cases, the charge pumpis in fluid communication with the reservoir 71 to obtain fluidtherefrom. In FIG. 4B, the reservoir 71 is also optionally shown to befluidly coupled to the case train line connection of both the variabledisplacement pumps 35 and the fixed (or variable) displacement motor 7.

With reference to FIG. 4C, a control scheme 200 is shown that can beused to control one or more of the embodiments, and in particular thepropulsion system 100A shown in FIG. 4A. The control scheme 200 startswith a desired attitude for the aircraft as an input from a computer ora pilot. The desired attitude is computationally combined with signalsfrom attitude sensors (e.g., yaw, pitch, and roll sensors), and fed tothe flight control computer to generate the desired pump displacementfor the variable displacement pumps to correct the error between thedesired and the actual attitude due to the change of the desiredattitude and the disturbance (e.g., a gust of wind or the shift of thecenter of mass). The actual attitude with error is thus sensed with theattitude sensors (described above) and mathematically measured againstthe desired attitude. The flight control computer generates theassociated control signals for each of the variable displacement pumps(e.g., 35, 36, 37, and 38 shown in e.g., FIG. 2A). These signals are fedto displacement control devices (e.g., swash plate control devicesdescribed above) which thereby control the rotational speed of thepropeller. Selective rotational speed of each rotor then change theactual attitude to reduce the attitude error.

With reference to FIG. 5, another aspect of the present disclosureincludes an open loop speed control implementation of the propulsionsystem. A flight control computer 78 provides control signals todisplacement control devices 70, 75, 76, and 77 (e.g., swash platecontrol devices described above) which in turn control displacement ofthe variable displacement pumps 35, 36, 37, and 38 driven by the engine48 via a run-through driveshaft 50. The variable displacement pumps 35,36, 37, and 38 are coupled and deliver flow to fixed displacement motors7, 8, 9, and 10, through high pressure hydraulic lines 39, 41, 43, and45.

With reference to FIG. 6A, a schematic of a propulsion system 300Aaccording to the present disclosure is provided that can be used inconjunction with one or more of the embodiments disclosed herein. Thepropulsion system 300A of FIG. 6A is a closed circuit with a close-loopspeed control provisions. The embodiment shown in FIG. 6A includes aspeed sensor 73 and speed controller 74 providing a closed loop systemfor controlling speed of the fixed displacement motor 7 and the rotor 3.In another aspect of the present disclosure, a pilot can replace theflight control computer 78. Alternatively, the variable displacementpump 35 according to one embodiment of the present disclosure includes amechanical feedback device for its displacement that can be used todetermine actual displacement of the variable displacement pump 35 vs.desired displacement. According to another aspect of the presentdisclosure, the variable displacement pump 35 contains an electricsensor to measure the displacement and a micro controller to compare theactual displacement to the displacement commanded from the pilot orflight control computer and to adjust the pump displacement accordingly.

With reference to FIG. 6B, a schematic of a propulsion system 300Baccording to the present disclosure is provided that can also be used inconjunction with one or more of the embodiments disclosed herein.Differences between FIGS. 6B and 6A include a charge pump 85 and valve87. According to the embodiment shown in FIG. 6B, a charge pump 85 isused with each variable displacement pump 35; or in some cases, a chargepump 85 is shared by a number of variable displacement pumps 35. Thecharge pump 85 provides the hydraulic power for the displacement controldevice 70 of the variable displacement pumps 35. Valve 87 isolates thecharge pump 85 from the hydraulic motor and prevents unwanted rotationof the rotor. In any of these cases, the charge pump is in fluidcommunication with the reservoir 71 to obtain fluid therefrom. In FIG.6B, the reservoir 71 is also optionally shown to be fluidly coupled tothe case train line connection of both the variable displacement pumps35 and the fixed (or variable) displacement motor 7.

With reference to FIG. 6C, a control scheme 400 is shown that can beused to control one or more of the embodiments, and in particular thepropulsion system 300A shown in FIG. 6A. The control scheme 400 startswith a desired attitude for the aircraft as an input from a computer ora pilot. The desired attitude is computationally combined with signalsfrom attitude sensors (e.g., yaw, pitch, and roll sensors), and fed tothe flight control computer to generate the desired speed for each ofthe rotors to correct the error between the desired and the actualattitude due to the change of the desired attitude and the disturbance(e.g., a gust of wind or the shift of the center of mass). The actualattitude with error is thus sensed with the attitude sensors (describedabove) and mathematically measured against the desired attitude.Associated sensors for each rotor provide feedback signals that can becomputationally added (compared) to the commanded rotor speed. Thiscomparison provides a way to compensate for the speed of each rotor bycontrolling the variable displacement pumps. The flight control computergenerates the associated control signals for each of the variabledisplacement pumps (e.g., 35, 36, 37, and 38 shown in e.g., FIG. 2A).These signals are fed to displacement control devices (e.g., swash platecontrol devices described above) which thereby control the output foreach variable displacement pump which can then generate selective rotorspeed control. Selective rotational speed of each rotor then change theactual attitude to reduce the attitude error.

With reference to FIG. 7, another aspect of the present disclosure whichis based on a closed loop control system is presented. The flightcontrol computer 78 generates command signals for motor speed and eachmotor speed is close loop controlled by the speed controllers 74, 79,80, and 81, and the displacement control devices 70, 75, 76, and 77(e.g., swash plate control devices described above) using the signalfrom the speed sensor 73, 82, 83, and 84. According to another aspect ofthe present disclosure, the speed controllers 74, 79, 80, and 81 areintegrated into the flight control computer 78. According to yet anotheraspect of the present disclosure, the speed controllers 74, 79, 80, and81 are integrated into the variable displacement pumps 35, 36, 37, and38.

It should be appreciated that each motor described herein can be a fixedor variable displacement motor. In the variable displacement embodimentsof the motor, the displacement of the motor is changed in order tofurther change the rotor speed based on the corresponding pump flow.

Those having ordinary skill in the art will recognize that numerousmodifications can be made to the specific implementations describedabove. The implementations should not be limited to the particularlimitations described. Other implementations may be possible.

1. A hydraulic propulsion system, comprising: one or more input interfaces configured to receive mechanical power from a power source; four or more variable displacement pumps coupled to the one or more input interfaces adaptable to generate a controlled variable quantity of fluid to be pumped out of each of the variable displacement pumps in response to a control input from a corresponding control interface; and four or more positive displacement motors each fluidly coupled to a corresponding variable displacement pump and configured to receive the pumped fluid, wherein each motor is configured to be mechanically coupled to one or more aerodynamic rotors of a multi-rotor vertical take-off and landing aircraft to control thrust and attitude.
 2. The system of claim 1, wherein the power source is one or more internal combustion engines.
 3. The system of claim 1, wherein the power source is one or more electric motors.
 4. The system of claim 1, wherein the power source is one or more turbine engines.
 5. The system of claim 1, wherein the positive displacement motors are fixed displacement motors.
 6. The system of claim 1, wherein the positive displacement motors are variable displacement motors adapted to further change the rotor speed for the corresponding pump fluid flow and wherein the motor displacement is controlled by a motor displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof.
 7. The system of claim 1, wherein the quantity of fluid to be pumped out of each of the variable displacement pumps is controlled by a pump displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof.
 8. The system of claim 1, wherein the control input is provided from one of a flight control computer, a pilot, and a combination thereof.
 9. The system of claim 1, further comprising a flight control computer coupled to each of the variable displacement pumps and configured to control the quantity of fluid to be pumped from each.
 10. The system of claim 9, further configured to receive a signal corresponding to the speed of each motor and to provide the speed information as speed feedback signals to the flight control computer.
 11. The system of claim 10, further comprising a closed-loop control arrangement using the speed feedback signals.
 12. The system of claim 10, further configured to receive a signal corresponding to the displacement of each positive displacement motors and to provide the displacement information as displacement feedback signals to the flight control computer.
 13. The system of claim 12, further comprising a closed-loop control arrangement using the displacement feedback signals.
 14. The system of claim 10, the flight control computer further configured to receive signals corresponding to one or more of position, attitude, and motion of the aircraft and control fluid flow from the variable displacement pumps accordingly to achieve a desired position, attitude and motion of the aircraft.
 15. The system of claim 7, further comprising a charge pump adapted to provide power for the pump displacement control device.
 16. The system of claim 1, further comprising a fluid cooling device adapted to cool fluid used therein.
 17. The system of claim 1, wherein the variable displacement pumps are coupled to each other in series manner.
 18. The system of claim 1, wherein the variable displacement pumps are coupled in pairs in a series manner, and each pair is coupled to at least one other pair in a parallel manner.
 19. The system of claim 18, wherein each pair is coupled to a dedicated power source.
 20. The system of claim 18, wherein each pair is coupled to the one or more input interfaces.
 21. (canceled)
 22. (canceled)
 23. The system of claim 1, wherein the fluid is a compressible fluid.
 24. The system of claim 23, wherein the compressible fluid is air.
 25. The system of claim 1, wherein the fluid is an incompressible fluid.
 26. The system of claim 24, wherein the incompressible fluid is one of hydraulic oil, water, fuel, antifreeze, and a combination thereof. 